The present invention relates to a transmission electron microscope and an image observation method using it and particularly relates to a transmission electron microscope capable of observation of an image not affected by drift and an image observation method using the transmission electron microscope.
A transmission electron microscope (TEM) is an apparatus using electron beams for directly observing a fine structure of a specimen and has far higher resolving power than that of an optical microscope so that the TEM can perform atomic-level structure analysis. The TEM is an apparatus indispensable nowadays for the medical and biological field and the metallic and semiconductor material field. Because a technique for observing a specimen while heating/cooling it has been developed, the TEM has been highly recognized to be not only an observation apparatus but also an on-site experimental apparatus.
To perform atomic-level structure analysis, an image is recorded with such magnifying power that a structure can be recorded on an atomic level. Image drift is an issue on this occasion. If an image is recorded in the condition that image drift occurs, the image flows in a drift direction to restrict structure analysis remarkably. Particularly when the specimen is observed while heated/cooled, the influence of image drift becomes remarkable.
A chief cause of image drift is thermal, mechanical and electrical stability. A factor of thermal instability is shape distortion due to temperature changes of a mirror body, a specimen holder and a specimen itself. Causes of the temperature change of the mirror body are change in electric current applied to an electron lens, change in temperature of lens-cooling water, change in room temperature, change in air-conditioning wind direction, and so on. Causes of the temperature change of the specimen holder are deformation at the time of observing the specimen while heating/cooling the specimen, and so on. Causes of the temperature change of the specimen itself are deformation, damage, and so on, due to change in density of applied electron beams.
Factors of mechanical instability are relaxation of stress confined in a specimen manipulation mechanism, and mechanical variation. In the former, stress is high just after movement of the specimen but is relaxed slowly. The latter occurs in such a manner that force for suppressing the specimen manipulation mechanism under atmospheric pressure out of a vacuum changes when air pressure around the apparatus varies. In addition, when floor vibration has a frequency (usually about 1–2 Hz) of resonance with the mirror body, the mechanical variation appears as vibration of the image.
A factor of electrical instability is electrical variation. When the intensity of magnetic field around the mirror body changes, electron beams are deflected so that the image moves.
To suppress image drift, a lot of know-how has been accumulated until now. The simplest and effective know-how is to wait for termination of drift. To perform observation/photographing particularly hating image drift, a measure to supply the apparatus with electricity continuously from the previous day to pay attention to thermal and mechanical stability is generally performed. High-precision equipment is often installed so that the temperature change of an air-conditioner or cooling water becomes as small as possible. The wind from the air-conditioner must be provided so as not to be blown against the mirror body directly. A method of reducing heat change due to current change of the electron lens has been disclosed in Japanese Patent No. 2,585,833. This is a method in which the circumference of an excitation coil is evacuated to a vacuum to thereby reduce heat conduction. An example for suppressing thermal deformation of the specimen holder has been disclosed in Japanese Patent No. 3,314,422. This is a method in which parts high and low in thermal expansion coefficient are combined to cancel the expansion and contraction of the specimen holder. Reduction in applied current density or fixation of the applied region is effective in preventing deformation and damage of the specimen itself.
To reduce mechanical variation, there is the case where an antivibration pedestal is set. Antivibration pedestals are classified into active antivibration pedestals and passive antivibration pedestals. The former detects mechanical variation and gives antiphase variation to a rack on which the apparatus is set. The latter shifts a resonance point with the mirror body to make it difficult to transmit floor vibration directly. A method in which the drift amount of a specimen manipulation mechanism due to change in air pressure is measured in advance so that a piezoelectric element incorporated in the specimen manipulation mechanism is used for canceling variation has been disclosed in JP-A-2004-55300. To reduce electrical variation, there is the case where a magnetic field canceller is set. This detects variation in magnetic field and gives antiphase variation to the mechanism.
To shorten the photographing time as extremely as possible is also effective. While the photographing time is suppressed to be short, the density of the electron beam applied onto the specimen is increased or the developing time is elongated instead.
A method of performing photographing while correcting drift has been proposed. In a method disclosed in Japanese Patent No. 3,454,052, because element distribution images are accumulated in a scanning transmission electron microscope (STEM) for a long time, difference between STEM images output at the same time is detected and a primary electron beam is deflected from moment to moment to thereby correct drift. As another method, there has been proposed a method in which drift speed is measured in advance so that a specimen is moved at a constant speed by a piezoelectric element attached to a specimen stage to thereby cancel drift. A method in which an image shift coil is controlled to cancel drift generated at the time of through-focusing has been proposed in Ultramicroscopy, Vol. 54, pp. 250–260 (1994). In this paper, there has been also proposed a method in which a TEM image is recorded in a video recorder in advance so that each frame is converted into a digital image and that drift is corrected as image data. A method in which view field difference between a plurality of TEM images is calculated to thereby correct specimen drift has been described in JP-A 2001-118535. A method in which a TEM image is picked up by a TV camera disposed above a fluorescent screen so that drift is detected and corrected has been described in JP-A-7-272665.
The simplest method is to wait for termination of specimen drift but a long time is taken and it is inconvenient to wait for termination of drift whenever the position of the specimen is changed. If preparation cannot be made from the previous day, a considerably long time is required for termination of drift. The magnetic field canceller, the high-precision air-conditioner and the cooling water equipment are expressive. The manipulation mechanism using the piezoelectric element for correcting change in air pressure cannot be fit to drift caused by other factors. If photographing is performed in a short time and development is performed for a long time, the negative film becomes so “hard” that the image becomes rough. Even in a slow scan CCD (SSCCD), signal-to-noise ratio (S/N) is lowered because of shortage of signal amount. If a high-luminance electron gun is mounted in order to obtain sufficient degree of darkness, there is the possibility that the specimen may be damaged because of high applied electron beam density. Drift correction of the element distribution image obtained in the STEM cannot be applied to the TEM image. The method of correcting drift by the manipulation mechanism using the piezoelectric element is not effective for drift out of uniform rate. The method of correcting drift due to through-focusing by the image shift coil cannot be applied to general drift. The method of photographing the TEM image as a video image cannot be applied to the photographic film as well as much labor is taken for correction. The method of calculating view field difference between the plurality of TEM images and correcting specimen drift cannot correct drift generated while each TEM image is photographed. The method of taking the TEM image in by the TV camera disposed above the fluorescent screen and detecting and correcting drift cannot detect drift because the TEM image does not appear above the fluorescent screen when the TEM image is recorded.